Which part of Eastern Europe? It’s 1800km from the western border of Poland to the eastern border of Russia, FYI.
It really doesn't matter, the OP's dog probably wasn't on that truck but a 100 others were and dogs don't have many degrees of separation. The first thing that any vet/dog trainer/experienced owner will tell you is that once a dog has its vaccinations,* it needs to be socialised.
(*Many will now advise doing this with large breed dogs before they are fully vaccinated, as they are more likely to be destroyed due to aggression stemming from a lack of socialisation, than illness.)
Dog socialisation is like a real life game of Guess Who? You go out and find men with hats, people of different ethnicities, people using various mobility aids and as many dogs as you can find, while the puppy is still in its socialisation window.
Dog have no reticence about getting up close and personal with each other and viruses spread like wildfire. When I lived in Surrey and used the local park, we had an outbreak of infectious red eye and the local vet became so overwhelmed, that he was treating dogs in the carpark, he claims to have treated a hundred cases. His was one of four practices in the town.
One hundred vectors arriving in Dover is more than enough to spread any dog-born virus right through the UK.